The present invention relates to common-mode feedback (CMFB) circuits used in differential amplifiers, and in particular to such circuits for driving an unknown capacitive load.
In some applications it is desirable to have an amplifier that is capable of driving any capacitive load. Since this load is not known in advance, it is often difficult to construct a differential amplifier composed of a differential feedback loop and a common-mode feedback loop, both which are stable with any load. Therefore, it is desirable to have a configuration where the dominant pole is set only by the output load capacitance for both the common-mode and differential feedback loops. In order to have a good power supply rejection ratio (PSRR) it is desirable to have a fully differential buffer structure. To stabilize the common-mode output level of a differential buffer, a common-mode feedback (CMFB) circuit may be used. Typically, this CMFB loop contains at least two voltage amplifiers, making the CMFB loop a multi-stage/multi-pole type design. This results in a potential stability problem depending on the load conditions of the buffer.